Recommended Use. In general, the 3% interference in all thread sizes is recommended for harder, less malleable materials such as cast iron. 5% interference is recommended for 3/8" (M10) diameter and up for softer materials such as aluminum. The 8% interference is recommended for sizes #10 (M4) through 5 16 " (M8), for use in aluminum. All interferences are based on the use of tap drill sizes which achieve 75% of internal thread. Sizes smaller than #10 are available. Metric sizes available.